This invention relates to catalyst-coated foundry sand, and processes for making and using same.
Molds for casting molten metals comprise several mold members working together to define the internal and external shape of the casting. Such members include core members for forming and shaping interior cavities in the casting, as well as cope/drag/shell members for forming and shaping the exterior of the casting. Such mold members are made by (1) mixing sand with a binder, (2) introducing (e.g., blowing) the binder-sand mix into a mold containing a pattern for shaping the sand-binder mix to the desired shape, and (3) curing/hardening the binder in the pattern mold to fix the shape of the mold member. During casting, molten metal is poured into or around the mold member(s), and allowed to solidify. The internal cores are removed from the casting by (1) hammering, (2) shaking, (3) heating to oxidize and crumble the binder, or (4) combinations thereof.
A variety of polymers are commonly used as binders in the so-called xe2x80x9chot-boxxe2x80x9d, xe2x80x9cwarm-boxxe2x80x9d, xe2x80x9ccold-boxxe2x80x9d and xe2x80x9cno-bakexe2x80x9d techniques for making such mold members, as is well known to those skilled in the foundry art. Gelatin (a biopolymer) is also used as a binder as taught by Siak et al U.S. Pat. No. 5,320,157 and Siak et al U.S. Pat. No. 5,582,231, which are herein incorporated by reference. Gelatin is desirable because it is water soluble, environmentally benign, and low cost. Moreover, less heat is required to break the bonds of the gelatin""s protein structure and oxidize the binder than is required for the other polymer binders. As a result, cores made with gelatin break-down readily from the heat of the molten metal alone, and thereby permit ready removal of the core sand (e.g., by shaking) from the casting with a minimum of additional processing. Powdered oxidation-promoting catalysts (i.e. ferric oxide, ferric phosphate and/or ferric pyrophosphate) have been added to the gelatin-sand mix to promote breakdown of the gelatin binder at aluminum casting temperatures. Following casting, the core sand is baked to remove charred and uncharred gelatin and reconstituted by adding fresh gelatin, as well as additional powdered catalyst to make up for catalyst xe2x80x9cfinesxe2x80x9d (small particles) lost in handling and processing the mix. This process requires monitoring of the Fe2O2 content of the sand to determine how much make-up Fe2O2 is required as well as measuring the needed amount and mixing with the sand. It would be desirable to simplify the reconstitution process by eliminating the need to have to determine how much catalyst is needed as well as have to handle and mix the catalyst powders with the binder-sand every time a new batch of sand is prepared.
Moreover, in the so-called xe2x80x9cLost Foamxe2x80x9d process, a fugitive foam pattern (e.g. polystyrene) is submerged in a bed of loose sand. Molten metal (e.g. Al) is poured onto the foam, which causes the foam to liquefy/vaporize and escape into the bed of sand where some of it remains as a residue. The metal fills the cavity left by the vaporized foam. The sand is reclaimed by heating in air to a temperature of about 760xc2x0 C. for about one hour to oxidize and remove the residue from the sand. It would be desirable if this reclamation process could be accomplished quicker and at a lower temperature.
The present invention contemplates foundry sand modified to (1) accelerate the thermal oxidation of polymer binders or residues in foundry sand for more effective break-down thereof, and/or (2) eliminate the need to have to add fresh catalyst powders to each batch of sand-binder mix. With the modified sand, cores can be readily removed from castings, and polymer residues readily removed from sand during reclamation thereof.
According to one aspect of the invention there is provided a reusable foundry sand whose individual grains are coated with an adherent layer of an oxidation catalyst that promotes the thermal oxidation of polymers mixed with the sand. The catalyst preferably comprises ferric oxide (most preferred) and/or cupric oxide. Catalyst loadings of less than about 0.1% by weight of the sand is all that is needed to obtain the benefits of the invention though higher amounts may be used.
According to another aspect of the invention, there is provided a method of preparing foundry sand to promote the thermal oxidation of polymers mixed therewith wherein the sand grains are (a) coated with a solution of a precursor of an oxidation-promoting catalyst selected from the group consisting of ferric oxide and cupric oxide, (b) dried to deposit the precursor on the grains, and (c) heated sufficiently to convert the precursor to the catalyst. The precursor is preferably selected from the group consisting of ferric and cupric salts, such as ferric chloride (most preferable), ferric phosphate, ferric pyrophosphate, ferric oxalate, cupric oxalate and cupric chloride, and will have a loading, after drying, of less than about 0.3% by weight of the sand.
According to another aspect of the invention, there is provided a method of making an aluminum casting comprising the steps of forming a mold part from reusable polymer-bonded foundry sand whose grains are precoated with an adherent layer of an oxidation-promoting catalyst, and casting the aluminum against said mold part so as to shape the aluminum and heat the mold part sufficiently that the catalyst promotes the oxidation of the binder and crumbling of the mold part. Preferably, the binder comprises gelatin, and the catalyst comprises ferric oxide (most preferable) and/or cupric oxide.
According to another embodiment of the invention, there is provided a method of making a casting comprising (a) embedding a fugitive polymeric pattern (e.g. polystyrene) in a bed of foundry sand whose grains are coated with a layer of an oxidation-promoting catalyst, (b) pouring molten metal onto the pattern in the sand bed to vaporize the pattern and cause it to migrate into and contaminate the sand with residue from the pattern, (c) removing the casting from the bed, and (d) reclaiming the sand for reuse by heating the sand sufficiently for the catalyst to promote the oxidation and removal of the residue from the sand quicker and at a lower temperature than would be possible without the catalyst layer.
According to still another aspect of the invention, there is provided a method of making a casting comprising the steps of (a) forming a core from polymer-bonded foundry sand whose grains are precoated with a reusable adherent layer of an oxidation-promoting catalyst, (b) casting molten metal about the core so as to shape the inside of the casting, (c) allowing the casting to solidify, and then (d) heating the casting sufficiently for the catalyst to promote the oxidation of the polymer for easy shakeout of the core from the casting.
A particular advantage of the present invention is that the catalyst-coated sand can be used repeatedly without substantial loss of catalyst and without having to add fresh catalyst powder to the mix.